A few older U.S. metropolitan areas still rely heavily on old cast iron pipes, which are more prone to leaks than plastic or protected steel are. For example, half of Philadelphia's pipes are cast iron.

Most systems that rely on cast iron pipes report higher amounts of fuel lost—what governments and the gas industry calls "unaccounted for" gas—than the national average of 1.3 percent for large natural gas systems.

On average, cast iron pipes make up just 3.5 percent of large distribution systems in U.S. cities. Here's a look at the systems with the highest risk of leaks due to cast iron pipes: